In the manufacture of lightweight paper grades, such as tissue, newsprint and fine paper, a continuous web or strip of paper is formed and in some cases pressed in the wet end of the machine, then dried on a series of steam-heated drying cylinders. The wet web is pressed directly onto at least some of the cylinders by tensioned, permeable fabrics or felts.
A conventional double-felted, two tier dryer group is shown in FIG. 1. The two tier, double felted dryer group, generally designated 10, includes internally steam-heated dryer cylinders 12, 14 and 16 arranged as an upper tier, generally designated 18, and similar dryer cylinders 20, 22 and 24 arranged as a lower tier, generally designated 26.
The rolls 30 and 32 are located closely adjacent to and between adjacent dryer cylinders of the upper tier 18. The rolls 34 and 36 are located closely adjacent to and between adjacent dryer cylinders of the lower tier 26.
In the double-felted, two tier dryer group of FIG. 1, the wet paper web shown as a dashed line W is threaded and travels around the bottom of one dryer cylinder such as 20 in the bottom tier 26 of dryers, where the top of the web W is dried; then around the top of one dryer cylinder such as 12 in the top tier 18 of dryer cylinders, where the bottom of the web W is dried; then around the next dryer cylinder 22 in the bottom tier 26 of dryer cylinders; and so forth in a generally serpentine or up and down fashion. The directions of rotation of the dryer cylinders in FIG. 1 dictate that the web W is working its way from left to right in that machine.
The portions of the web W passing between the upper and lower tiers 18 and 26 of dryer cylinders form long, unsupported "open draws" as shown at the points 40, 42, 44, 46 and 48. The majority of the water vapor that leaves the sheet or web is released in these open draws. As it passes through the double-felted, two tier dryer group, the web W is dried alternately on each side. This two-sided drying tends to inhibit curl.
Problems have been experienced during operation of conventional double-felted, two tier dryer groups. For example, the sheet or web flutters as it moves through the open draws. This problem particularly occurs in the "wet end" of the dryer, where the web is still quite wet. A fluttering web frequently breaks on the machine. A web break is expensive and time-consuming to correct. Even an occasional web break is a very big problem. It can damage the felt and even the machine, and it inevitably causes production of paper to stop until any necessary repairs can be made and the web can be re-threaded in the running machine.
While a web break is being corrected, a web of undried paper as wide as the machine (often about 30 feet or nine meters wide) and miles (several km.) long is formed and must be collected, broken up, mixed with a much larger quantity of water, and recycled in the paper machine. The web must then be re-threaded in the machine while it is running.
Machine speeds, and thus the amount of paper a machine could produce, were limited by the need to avoid an excessive number of web breaks by keeping the web speed low enough to minimize its flutter in open draws. Even after taking this precaution, web breaks were a common occurrence.
Also, in conventional double-felted, two tier dryer groups, problems are caused by cross-directional sheet shrinkage and inefficient ventilation of evaporated water. Additionally, conventional double felted two tier dryer groups typically require threading ropes in order to thread a tail of the web.
Some of the problems with sheet flutter, sheet shrinkage, and vapor ventilation have been solved by using one or more top-felted single tier dryer groups in the wet end of the dryer.
Top-felted single tier dryer groups are arranged much like the top tier 18 of dryers, rolls 30 and 32, and top felt of FIG. 1. The primary difference is in how the web is threaded through the dryer group. Instead of going back and forth between two tiers of dryer cylinders, in a single tier dryer group the web and supporting felt follow the same path throughout the group. The web W and felt are wound together about one dryer cylinder with the paper web facing the dryer cylinder and the felt on the outside, then around a counter roll (typically a vacuum cylinder or other arrangement to keep the web on the felt) with the felt facing the counter roll and the paper on the outside, then to the next drying cylinder in sequence. The same side of the web contacts each dryer in the single tier group.
For example, in the top-felted single tier arrangement of FIG. 4, the dryer felt, with the web beneath it and following the same path, winds over the dryer cylinder 81, under the vacuum roll 88, over the dryer cylinder 82, under the vacuum roll 89, over the dryer cylinder 83, under the vacuum roll 90, over the dryer cylinder 84, under the vacuum roll 91, over the dryer cylinder 85, under the vacuum roll 92, and over the dryer cylinder 86. Each cylinder 81-86 directly contacts and thus preferentially dries only the bottom of the web.
Dryers as shown in FIG. 2 including one or more top-felted single tier dryer groups 60, 62, and 64, followed by two or more two-tier double-felted dryer groups 65, 66, and 67 at the dry end, like the one shown in FIG. 2, have been sold by Beloit Corporation under the trademark "BEL-RUN". Each top-felted single tier dryer in the groups 60, 62, and 64 only directly contacts one side of the web --specifically, the bottom side--with the heated surfaces of the drying cylinders.
It is well known that the sheet tends to curl toward the last side of the web to be dried, at least in laboratory studies. Therefore, a dryer consisting entirely of top-felted groups such as 60-64 would directly dry only one side throughout, and would severely curl the paper toward that side. Those skilled in the art have believed that preferentially drying the web on one side for very long without periodically compensating by drying directly on the other side would create an imbalance in drying on the respective sides of the web, leading to a problem with sheet curl.
In the dryers represented by FIG. 2, this problem was addressed by limiting the series of top-felted single tier groups to the wet end of the machine, and following the single tier dryer groups with double tier groups 65-67 which reverse the sheet with each succeeding dryer cylinder and thus avoid curl. A typical dryer including single tier sections had approximately 40 percent of the dryer cylinders in top-felted single tier groups, and the remaining dryer cylinders are two tier, double felted dryer cylinders. In FIG. 2, 14 of the 36 dryer cylinders--39%--are in the single tier groups 60-64, and 61% of the dryer cylinders are in the two-tier groups 65-67.
Subsequently, the dryer illustrated in FIG. 3 was developed, consisting entirely of single tier sections. The problem of one sided drying was addressed in these all-single-tier machines by providing alternating top-felted groups (60B, 72, and 76) and bottom-felted or inverted groups (70, 74). Such dryers have been sold, for example, under the trademark "BEL-CHAMP" by Beloit Corporation.
In this arrangement, a top-felted group like 60B preferentially dries the bottom of the web and a bottom-felted group like 70 preferentially dries the top of the web. Thus, any one single tier dryer group preferentially dries just one side of the web.
Concern about sheet curl led paper dryer builders to dry alternate sides even in the very early stages in the alternating single tier dryers. For example, the #3 machine at CTS, Duino, Italy, was designed with the first three dryer cylinders top felted, the next three bottom felted, and the following three top felted. These first three groups were made small to ensure alternate-sided drying would be started in the very early stages of the drying process.
According to conventional thinking, therefore, the sheet must be reversed periodically by contacting it alternately with top-felted and bottom-felted dryer cylinders or groups to avoid curl by frequently reversing the side of the web being dried.
Top-felted single tier dryer groups are preferred over bottom-felted single tier dryer groups, particularly in the wet end of the machine. Although web breaks are infrequent in single tier groups, when the web does break, if it then wraps around a bottom felted dryer cylinder, the wrapped paper cannot be easily dumped into the basement, as the felt underlies the web throughout the group. (The "basement" of a paper machine is the open space beneath the machine. The "broke" or displaced, partially made paper produced by the machine is collected in the basement for recycling.) Rather, the broke must be manually removed from the bottom felted dryer group by a worker. The manual removal of broke is time-consuming, and often must be done while the machine is stopped.
Top-felted groups are open beneath the web, as the felt is above the web in such a group, so broke automatically goes into the basement when the web breaks, or can be easily diverted there.
Thus, adding bottom-felted single tier groups like 70 and 74 in FIG. 3 to avoid curl has caused other problems in alternating single tier arrangements.
Another difficulty with the entirely single tier dryer is that none of its dryer cylinders are stacked vertically by providing upper and lower tiers of dryers. Conventionally, the successive dryer cylinders are in a generally horizontal arrangement rather than in a two tier arrangement. Thus, the machine commonly is longer than earlier machines which have the same number and size of dryer cylinders. This all single tier arrangement can have the disadvantage of requiring more interior floor space than a double tier dryer group, even though each dryer cylinder of a single tier arrangement commonly has a higher drying capacity than one dryer cylinder of a two tier arrangement.
The alternating single tier figuration also commonly requires two vacuum rolls at each of the transfers from the last dryer of one dryer group to the first dryer of another group, compared to an all-top-felted single tier arrangement which requires just one vacuum roll near the transfer or a double-felted two-tier group which may require none.